As used herein, the term "high pressure" is used to denote pressures in excess of 20,000 psi. Pressures in excess of 20,000 psi are experienced in a number of technical areas including, but not limited to, high pressure waterjet cutting. Although this particular application will be used as an example throughout this specification, it should be understood that the described invention is not so limited.
High pressure water jet cutting systems typically compress water, or other liquid, to working pressures in excess of 20,000 psi. Some of the systems, such as those manufactured by Flow Systems, Inc. in Kent Washington, operate at working pressures in the range of 30,000 to 35,000 psi. At these working pressures, the hoses preferably have burst pressures in the order of 60,000 psi.
These high pressure hoses comprise an axially-extending, nonmetallic liner of generally annular cross-section generally circumventing the hose's fluid-conducting passageway. The liner is typically formed from a thermoplastic such as Nylon or Delrin.sup.1. The liner is generally circumvented by an axially extending reinforcement layer, formed by multiple wraps of steel wire or Kevlar.sup.2 filaments. Each wrap may circumvent the exterior of the liner at a different angle to the hose axis than the others. FNT 1. A trademark of E. I. du Pont de Nemours and Company for synthetic resinous plastic material. FNT 2. A trademark of E. I. du Pont de Nemours and Company for manmade fibers.
The hose is coupled to the source or destination of the conducted fluid by means of a generally tubular end fitting comprising a shank and a sleeve. The shank is a generally tubular member formed from stainless steel or other suitable metal, and having an internal passageway in fluid communication with the hose's passageway. Its leading end portion, referred to as the nipple, is inserted within the end of the hose. A generally tubular metal sleeve circumvents the nipple and overlying hose fitting and is swaged or crimped radially inwardly to effect a seal between the shank, hose, and sleeve.
The sealed fittings described herein are particularly directed to "large-diameter" high pressure hoses; i.e., those having fluid-conducting diameters of 0.3 inches or more. While the sealing of large diameter hose fittings at relatively low pressures has been comparatively uncomplicated, vis-a-vis the sealing of small diameter low pressure fittings, owing to the amount of space available for components of the seal, complications arise when attempts are made to seal a large diameter, high pressure fitting. In designing high pressure end fittings, as the term "high pressure" is used herein, problems which are not significant at lower pressures become significant. As the high pressure sealing arrangement becomes more complex the number of ways in which the seal can fail multiplies. There are two significant sources of sealing failures in large diameter, high pressure hoses. First, the hoses expand significantly under the high pressures described herein, owing to radial compression of the reinforcement layers. As the assembly's radial dimensions change, leaks develop within the sealing arrangement.
Second, the reinforcement layers have to be inwardly compressed by a significant amount during the swaging operation before they transmit a sufficient amount of that externally applied force to the nipple/liner interfaces to render the maintained interface pressure higher than the working pressure of the hose. The degree of required crimping gives rise to two additional problems. First, the crimping operation distorts the reinforcement layers both radially and axially; e.g., a 4 inch long sleeve when used with a 0.5 inch bore hose, may grow axially by 0.25 inches while the hose itself grows axially by 0.375 inches. The distortion of the reinforcement layers creates a region of uncertain strength on the hose adjacent to the fitting, and many hoses burst at a point within that region.
Second, the crimping process requires the crimp sleeve to be able to undergo plastic deformation with minimal springback when the crimping pressure is released. Accordingly, a low yield strength material is required. On the other hand, the sleeve must be able to maintain sufficient interface pressure so that the seal between the hose and fitting is maintained at working pressure. This, in turn, requires a high yield strength material. To meet the need for those two conflicting requirements, the most widely used materials have been 1040 mild steels and 316 stainless steels which have yield strengths in the range of 35,000-40,000 psi. As the working pressure of the conducted fluid is near that range, however, the repeated pressure cycling to which the hoses and fittings are routinely subjected in normal operation, leads to a yielding of the sleeve itself and to a consequential loss of interface pressure.
Additionally, a number of effective sealing techniques known for small diameter high pressure hoses prove inadequate when applied to large diameter hoses owing to the increased size of the seal components. The extremely high pressure of the conducted fluid translates into an exceedingly high force when acting upon a large surface area. Accordingly, an effective high pressure seal for large diameter hoses should preferably be formed by a minimal number of low surface area components.